1. Field of the Invention
The present invention relates to a semiconductor memory disk apparatus which comprises a volatile semiconductor memory and a backup device for saving data stored in the semiconductor memory when a power source is downed or turned off and, more particularly, to a method of immediately executing ar external input/output command during a restore operation of data from a backup device to a semiconductor memory and a semiconductor memory disk apparatus for realizing the method.
2. Description of Related Art
In recent years, an apparatus using a semiconductor memory as a kind of disk device, i.e., a semiconductor memory disk apparatus has been developed. Since a semiconductor memory disk apparatus of this type employs a semiconductor memory as its storage medium, it does not require a mechanical operation (unlike a magnetic disk or tape device), thus allowing a high-speed operation. However, a volatile semiconductor memory is used as the storage medium due to its cost. Therefore, the memory must be backed up when an AC power source is downed or turned off upon a power failure or the like. In the conventional semiconductor memory disk apparatus, a battery power source for backup when the power source is OFF is prepared. When the power source is OFF, data stored in the semiconductor memory is saved in a backup device using the battery power source.
In the semiconductor memory disk apparatus as described above, as shown in FIG. IA, when the power source is recovered, a restore operation for restoring the data saved in the backup device to the semiconductor memory is started contrary to the power OFF state. In this case, as shown in FIG. 1B, since the content of the semiconductor memory does not coincide with that in a backup device until the restore operation is completed, execution of an input/output command input from an external CPU is inhibited. The restore operation requires several minutes since the backup device normally comprises a magnetic disk device or a magnetic tape device. Therefore, it takes a long period of time until the CPU is available after the power source is recovered.